Samuel



S. O. HOFFMAN.

LAMP.

APPLICATION FILED Nov. 28,192 I Patented Aug. 15, 1922.

INVENTOR J'AMuEL 0. HQFFMAN.

. B I I x44 ATTORNEYS UNITED STATES PATENT OFFICE.

SAMUEL O. HUFFMAN, OF SAN FRANCISCO, CALIFORNIA.

LAMP.

To all whom it may concern Be it known that I, SAMUEL O. HOFFMAN, acitizen of the United States, and a resident of the city and county ofSan Francisco, State of California, have invented a new and useful Lamp,of which the following is a specification.

This invention relates to illumination and more particularly to lamps ofthe type shown in my ber 29, 1911 and No. 1,083,325 and No. 1,083,326,dated January 6, 1914, which consist of an incandescent (source, andmeans for returning the Infra-red radiation back to the source where itis absorbed, and converted into luminous radiation; the luminousradiation being continually allowed to escape.

In my present application, I accomplish the sparation and backwardreflection of the infra-red rays directly, by employing a suitablyconstructed thin film on the inner surface of the glass globe, no innertube or globe being necessary. The theory of the action of this filmdepends upon phenomena known to physicists, manifested as the colorsobserved in connection with thin films, e. g. the colors of soapbubbles. Briefly, a film oftransparent material, of the order ofthickness of a wave length of light, will refuse to transmit light of acertain wave lengthor lengths, and transmit in excess lightcomplementary to this wave length. This is due to the destructiveinterference between the portions of light reflected from the twosurfaces of the thin film. Since the theory of this action is wellunderstood, I will not elaborate it but simply state, the facts asabove. These thin films in general selectively reflect only a limitedportion of the spectrum.

Previous to my invention it had not been noticed that a film of acertain critical thickness will reflect practically all the wave lengthscomprising white light and transmit all Y the infra-red wave lengths;or, vice versa, transmit all the wave lengths comprising white light andreflect all the infrared, which of course constitutes the idealselective reflector.

vThe actual thickness of this film cannot be stated in units of length,as it varies with the nature of the material constituting the'thin film.

The thickness, however, may be defined in terms of Newtons rings: Takingthese as an index, the thickness of the film that may Patent-No.1,043,008 of Octoergy (depending upon at the preferably Specification ofLetters Patent. Patented Aug, 15, 1922 Application med November as,1921. Serial No. 518,256.

transmitted up to the extreme red, butfor the longer wave lengths anamount of endices of the thin film and its surrounding the refractiveinmedium) would be reflected back; that is a thickness equivalent to thewhite ring would still be a central black spot by transmitted light forthe infra-red waves, or a bright spot by reflected light, or statedotherwise, the infra-red waves would be reflected back.

As only a portion of the energy is acted upon by a single film, it wouldbe necessary to use a number of such films which the l l u a radiationtraverses in succession, or else usea film with a higher reflectingpower than the ordinary transparent film. Such a special film can beeasily constructed by dcpositing by suitable means 'a thin layer of somesubstance such as stibnite. on each surface of the film, (which ingeneral would consist of such a substance as collodion or celluloseacetate) or even using a thin film of stibnite. I mention stibnitebecause it is typical of a number of highly reflecting substances, suchas the metallic sulphides which same time are transparent, more or less.The final result of any of the above arrangements is that substantiallyall the light escapes freely. and substantially all the infra-redradiation is returned to the source. 7

Of course the thickness corresponding to any white ring can be used; forexample with a white central spot, there is a white ring in the secondorder. Also lightof a particular color, and even mono-chromatic lightcan be obtained by a suitable thickness of the thin films.

In practice, the source of radiationis concentrated as nearly aspossible in the center of a spherical globe, on the innersurface ofwhich is placed the thin films;

The drawing graphically shows on an enormously enlarged and exaggeratedscale, an electric lamp embodying the invention, in which A represents alamp globe which is preferably spherical, and on the inner surface ofwhich is applied the series of coatingsB, constituting'the selectivereflector. is the source of radiation of Waves of various length. On theinner coating-B, 1

represents a transparent material, such as collodion or celluloseacetate, etc., and 2--2 represents layers of a highly reflectingtransparent substance, such as stlbnite. Only one of these combinationfilms is shown, but a number are usually used. The idea briefly is tohave alternate layers of transparent substances, differing in refractiveindices so as to secure a reflection at the boundary.

I claim:

1. A lamp embodying a source of radiation of Waves of various lengths,with a selective reflector around the source embodying a plurality offilms and a thin intermediate film of the critical thicknesscorresponding to the first white ring of Newtons rings, whereby onlywaves of desired lengths escape and waves of other lengths are reflectedback to the source to be absorbed and converted into waves of a lengthwhich will not be returned by the reflector.

2. A lamp embodying an incandescent source of radiation of waves ofvarious lengths, a selective reflector around the same embodying arefractive body, the thickness of which is in conformity with the lawsgoverning the interference colors of thin films, such that it allowswaves of the desired length to escape freely but reflects the waves ofother lengths back to the source where they are converted into waves ofa length which will be permitted to escape from the lamp.

3. The combination with a source of incandescent radiation, of aselective reflector around the same, consisting of alternate layers oftransparent substances of differing refractive indices, each alternatelayer being of the critical thickness corresponding to the first whitering of Newtons rings.

4. In a lamp, the combination of an incandescent source and a selectivereflector around the same, said reflector consisting of a thintransparent film on each side of which is deposited a layer of a highlyreflecting substance.

In testimony whereof, I have hereunto set my hand.

SAMUEL O. HOFFMAN.

